None
Not Applicable
The present invention is related to the field of sensors and their associated sensor control circuitry, and more particularly to the mechanical packaging of systems such as gyroscopes and accelerometers that employ sensors and sensor control circuitry.
Many types of systems use sensors to detect the value of a property of a physical system and to generate a corresponding electrical signal representative of the sensed value. The electrical signals from sensors are commonly provided to electrical circuitry off chip or in adjacent locations on chip in order to enable a desired function to be performed. For example, inertial guidance systems use gyroscopes and accelerometers to monitor the orientation of an object in flight and the direction and magnitude of forces experienced by the object. The gyroscopes and accelerometers rely on electromechanical sensors that translate particular types of motion, such as rotational or linear acceleration, into corresponding electrical signals.
It has become possible to fabricate very small or micromechanical sensors out of silicon wafers of the type used in the manufacture of integrated circuits. Micromechanical sensors are comparable in size to integrated circuits (IC""s ). During fabrication of micromechanical sensors, silicon wafers containing numerous sensors are diced into individual sensors, and the sensors are individually mounted into packages of the type used for integrated circuits, such as sealed leadless ceramic chip carriers. The packaged sensor is then mounted on a printed circuit board along with an associated control circuit, usually an IC itself and packaged in a similar manner. The sensor and control circuit, along with other circuit components as required, collectively perform the desired sensing function.
One of the drawbacks of the above-described method of packaging sensor-based systems is that individual unprotected sensors must be handled during packaging operations, potentially reducing yield and increasing manufacturing costs. Also, the sensor package is generally significantly larger and more expensive than the sensor, so the package contributes notably to the dimensions and cost of the assembled printed circuit board. Also, mounting the sensor in a package such as a chip carrier limits how close the sensor can be placed to the control integrated circuit, which in turn can unnecessarily limit the electrical performance of the sensor-based system or increase the susceptibility to noise.
It would be desirable to improve the cost, size, and performance of systems employing micromechanical sensors and associated control circuitry.
In accordance with the present invention, a method of fabricating and packaging micromechanical sensors and their associated control circuitry is disclosed that eliminates the need for a separate carrier package for the sensor. The disclosed packaging method advantageously eliminates the handling of exposed sensors during packaging operations, and results in closer placement of the sensor and its associated control circuit, so that costs are reduced and greater system performance can be achieved.
In the disclosed fabrication method, a micromechanical sensor is fabricated on a semiconductor wafer, and a control circuit is fabricated on another semiconductor wafer. A cavity is etched on the back side of the control circuit wafer, the cavity being formed such that the raised portion of a sensor fabricated on the other wafer fits within the cavity when the wafers are brought together in an adjoining relationship. Through-holes are etched through the back side of the control circuit wafer to allow access to electrical contact points of the sensor circuit, and a patterned layer of metal is deposited to form electrical interconnections between the electrical contact points and termination points on the back side of the control circuit wafer via the through-holes. The termination points are arranged such that electrical contacts of the sensor contact the termination points when the wafers are placed in the adjoining relationship. The wafers are then cleaned and bonded together in the adjoining relationship. In a typical process the respective wafers contain multiple sensors and control circuits, respectively, and thus the bonded wafers are diced to yield individual bonded sensor-circuit pairs. The bonded pairs are then packaged in a known manner.
The fabrication method eliminates the need for separate packages for the sensor and control circuit. Also, processing is done at the wafer clean room level so that the handling of individual exposed sensors is eliminated. The sensor and control circuit in each bonded pair are placed in intimate contact, thus reducing overall size and improving electrical performance, and the pairs can be packaged using conventional integrated-circuit packaging technology.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description which follows.